1. Field of the Invention
The present invention relates to coating compositions, pressure-sensitive and heat-sensitive coatings and a process for preparing same.
Present pressure-sensitive copying systems are primarily of two types. One such system, which is typified by ordinary carbon paper, provides a mark under pressure by transfer at the areas of pressure of a complete mass of coating containing carbon black or other pigment. This waxy mixture is relatively soft and of low tensile strength and is, therefore, not resistant to scuff or offset. The mark obtained on the copy sheet is also prone to smudge and offset onto hands and clothing.
A second type of pressure sensitive copying system employs, on one side of a sheet, a continuous coating containing marking fluid inclusions. In accordance with this system, the paper of a record material is coated on one surface with a continuous film containing minute droplets of an oily marking fluid. In this type of copying sheet, the marking fluid may be coloured or it may be colourless but capable of forming a colour by a chemical reaction with another chemical upon being brought into contact with each other. Under the pressure of writing or typing, the coating ruptures and the droplets of marking fluid exude on to the copy sheet to make a mark by direct coloration or colour transfer or by forming a colour by chemical reaction with a coreacting chemical on the adjacent surface of the copy sheet. The difficulty with products of this type is that the surface of the coated sheet is susceptible to scuffing, abrasion and incidental rubbing which unavoidably causes rupture of the film, release of the marking fluid, and consequently smudge or offset or both. Also, on storage of the sheets, the coating material tends to dry out and crack, particularly when bended or folded, thus releasing the marking fluid through the resulting cracks to smudge or offset an adjacent copy sheet or the hands or clothing. In the case of such coatings containing a colourless marking fluid, this difficulty arises only when the ruptured film is on contact with a copy sheet or other source of the coreacting chemical. But this is a very real disadvantage since such copying materials are not only used, but usually stored in this condition.
2. Description of the Prior Art
In the past, for example, according to Canadian Pat. No. 627,609 to Schossberger et al, pressure-sensitive particles having a dark core and a frangible light-coloured pigment covering are described. Under pressure the covering breaks and the dark core is exposed. Moreover, the pigments contained in the covering as described in this patent are limited to light-coloured pigments, for example titanium dioxide.